In general, as a cutting tool for cutting a work material such as a metal material, there are those in which a cutting insert whose upper and lower surfaces are usable, namely, a double-sided cutting insert is attached to an insert pocket of a holder.
This cutting tool comprises the cutting insert and the holder. The cutting insert is the double-sided cutting insert (the negative type) in which an upper surface side cutting edge and a lower surface side cutting edge are disposed at the intersection portion between the upper surface and a side surface, and at the intersection portion between the lower surface and the side surface. This cutting insert is economical because when the upper surface side cutting edge in use is worn, the cutting insert is turned back to use the lower surface side cutting edge not yet used.
On the other hand, an insert pocket to which the cutting insert is attached is formed at a front part of the holder. The insert pocket comprises a bearing seat surface against which the lower surface of the insert abuts, and two bearing side surfaces against which the side surfaces of the insert abut. The insert pocket is opened into a front surface and a side surface of the holder, respectively.
However, the bearing seat surface of the insert pocket is made smaller than the lower surface of the cutting insert, as disclosed in Japanese Unexamined Utility Model Application Publication No. 1-175104. Therefore, when the cutting insert is attached to the insert pocket, the lower surface side cutting edge of the cutting insert is protruded from the front surface and the side surface of the holder.
In this state, if a work material is cut under conditions, for example, where chips having a large curl diameter are generated, there is the problem that the chips curled up to the lower surface of the cutting insert are contacted with the lower surface side cutting edge, and the lower surface side cutting edge is fractured.
This problem is noticeable when in the upper surface and the lower surface of the cutting insert, a breaker groove is formed more inside of the cutting insert than the upper surface side cutting edge and the lower surface side cutting edge, or when the upper surface side cutting edge and the lower surface side cutting edge have a sharp edge, or when the cutting insert has a small thickness. This problem also occurs when the bearing seat surface of the insert pocket is formed in substantially the same size as the lower surface of the cutting insert.